This invention relates generally to phase change memories.
Phase change memory devices use phase change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state, for electronic memory applications. One type of memory element utilizes a phase change material that may be, in one application, electrically switched between a generally amorphous structural state and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. The state of the phase change materials is also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until changed by another programming event, as that value represents a phase or physical state of the material (e.g., crystalline or amorphous). The state is unaffected by removing electrical power.
Conventional phase change memories require programming currents to convert the phase change memories between the different states. Desirably, these programming currents are kept as small as possible in order to reduce power consumption. Generally, a heater is positioned under a phase change material and the current through the heater is responsible for changing the state of at least an overlying volume of the phase change material. Unless considerable current is provided to convert a substantial region of the overlying chalcogenide, the converted region of reset or amorphous phase change material may be insufficient to prevent some current from passing past the converted material. The current flow at a small read voltage may be interpreted electrically as a low resistance state even though the region directly above the heater is amorphous. When a higher current is used to create a larger heated mushroom, the phase change material along these potential leakage paths is converted from crystalline to amorphous, allowing the cell to reach a completely reset state, but at the expense of considerable current consumption.
In addition, the transition from the set to the reset electrical state is fairly abrupt because the chalcogenide regions adjacent the heater periphery must be completed converted into a reset state to effect a large increase in the measured cell resistance. If these regions are not completely converted, then a low resistance state can still be read electrically. When these regions are entirely converted, a measured cell resistance climbs quickly to the reset resistance. This makes it challenging to adopt a multilevel programming technique since small changes in current can result in large and varying increments in cell resistance, depending on the amount of chalcogenide material which is converted at the heater periphery.
Thus, there is a need for better phase change memories for implementing multilevel programming.